Paper product and method of making

ABSTRACT

Paper having improved curl and cockle properties for water based, ink jet printing applications and a method of producing them. The paper contains at least 50 pounds per ton of a mono alkali metal salt of citric acid. The paper has a maximum Cockle Value of 0.25. It also contain a hexasulfonated optical brightener. The paper is also water fast and can be preprinted with offset printing inks. A method for making the paper.

RELATION TO OTHER APPLICATIONS

This application is a continuation-in-part of application Ser. No.10/743,846 and 10/744,856, both filed Dec. 22, 2003.

FIELD

The present invention is directed to printing paper, and the method ofmaking this paper.

BACKGROUND

Hardwood and softwood wood pulp fibers are used in the manufacture ofprinting paper and newsprint. These fibers are produced in a chemicalpulping process, either sulfate or sulfite, or in a mechanical pulpingprocess. Mechanical processes would include thermomechanical andchemithermomechanical. To form the printing paper or newsprint, thesehardwood or softwood pulp fibers and wet end chemicals are mixed withwater in the headbox of the paper machine to form a suspension of fibersis and chemicals. The wet end chemicals may include fillers such ascalcium carbonate and clay. The suspension of fibers and chemicals flowfrom the headbox onto a wire. The water is removed from the fibers andchemicals by both gravity and vacuum to form a wet web of pulp fibersinto which the chemicals are incorporated. The chemicals are throughoutthe sheet. The sheet may be pressed and dried to remove more water.

Starch, optical brightener additives and surface size may be placed onsurface of the sheet in a surface sizing step at the size press Some ofthe materials may enter into the web if the pressure of the nip at thepress is great enough.

Thereafter the web of fiber, wet end chemicals and other materials isdried by heat, calendered and rolled into rolls. The resulting productis referred to as an uncoated or lightly coated paper sheet or web.

The uncoated sheet may be coated in another application of one or morecoating layers placed on the sheet in an off-line coating operation. Theuncoated sheet passes through a coating station and a second dryingstation. It may pass through a second calendering operation. Theresulting product is referred to as a coated paper sheet or web.

Uncoated or coated printing paper has a basis weight of from 16 to 180pounds per 3300 square feet.

The application of high speed, variable printing is experiencingtremendous growth in the printing industry, displacing conventionaloffset printing for many applications. A digital printing technologysuch as web-fed ink jet printing presents new and different challengesfor the paper maker as the optimum surface physics and chemistry ofpaper for these printers are very different than those required forconventional offset inks.

High speed, ink jet printing is exceptionally challenging because itemploys aqueous inks and a great deal of water is placed on the papersurface during the printing process. In the process, these water basedinks may be applied at high coverage at paper speeds of 500-1,000 ft.per minute. It is difficult to completely dry the paper before the paperleaves the printer. If uncoated paper is used, the water from the inkpenetrates the sheet and disrupts the bonding between the paper fibers.This creates a deformation of the paper surface, which results inunacceptable curling, cockling, or puckering of the printed paper.

Standard desk top ink jet printers are increasing in speed and some ofthe same challenges are found when printing with these printers becauseof the water placed on the paper and the difficulty of completely dryingthe paper before it leaves the printer.

Wide printers have similar challenges because of the amount of ink andwater placed on the paper.

Because of these factors, special papers are used when the print jobrequires high levels of ink coverage. These special papers are coatedwith water-absorbent silica or swellable gel materials such as polyvinylpyrilodone, or combinations of these materials. Typically, thesematerials are applied by an off-line coating operation. The price ofthese materials and the off-line application significantly increases thecost of paper for these applications.

The optical density of the printed image is also of primary concern formany print jobs as high levels of ink are required to provide vivid,robust colors. This is known as high optical density. Uncoated papersare limited in the amount of ink they can tolerate because of theirtendency to curl and cockle. Thus more expensive coated papers aregenerally required when high optical densities are needed.

The inks are anionic. Highly cationic chemicals are usually added to thepaper in order to precipitate the ink and cause the ink to be waterfast.

Hexasulfonate optical brighteners are used to enhance the brightness ofthe paper and make it appear better visually. These optical brightenersare also anionic and the highly cationic chemicals will react with theoptical brightener also. This reduces the ability of the brightener tobrighten the paper.

Many of these papers are preprinted in an offset press before beingprinted in an ink jet system. An example would be placing a watermark orlogo on the paper. These inks are also anionic and a highly cationicchemical reacts with these inks also. This creates a problem in cleanupof the offset presses.

It is desirable to find a material that may be placed on an uncoatedpaper in a size press operation, that will reduce the cockle in paper sothat it may be used in high speed ink jet printing, make the ink waterfast so that it does not bleed when sprayed with water, and may allowthe paper to be pre-printed in an offset press.

SUMMARY

The present invention is directed to an uncoated paper usable with inkhaving a water content and which has a maximum Cockle Value of 0.25. TheCockle Value is used to determine the amount of cockle or water inducedcurt in the paper. An embodiment of the invention is an uncoated paperhaving a paper basis weight of 16 to 180 pounds per 3300 square feet anda maximum Cockle Value of 0.25.

An embodiment of the invention is a paper that has been treated with aat least 50 pounds per ton of paper with a mono alkali metal salt ofcitric acid, a mono sodium or potassium citrate, that is capable ofbeing added at the size press, blade coater or by a spray before theheated drying section. Another embodiment is a paper that has beentreated with at least 75 pounds of the citrate per ton of paper. Anotherembodiment is a printing paper that has been treated with up to 250pounds of the citrate per ton of paper. Another embodiment is a paperthat has been treated with up to 300 pounds of the citrate per ton ofpaper.

It has been found that a mono alkali metal salt of citric acid such asmono sodium or potassium citrate has enough cationicity to react withthe ink jet inks to cause them to be water fast. The mono alkali metalsalt of citric acid also causes the paper to have a Cockle Value of 0.25or less. The mono alkali metal salt of citric acid does not have enoughcationicity to react with an optical brightener, including hexasulfonateoptical brighters, or to react with offset printing inks. It provides apaper that is water fast,that has a reduced cockle and can be preprintedin an offset press.

In another embodiment of the invention the paper also contains a bindersuch as starch, ethylated starch, latex, polyvinyl alcohol, styreneacrylic acid or an ester in addition to the mono alkali metal salt ofcitric acid and tne low viscosity of the additive can be maintained.

In another embodiment of the invention the paper also contains aflorescent whitening agent, an optical brightener or a hexasulfonatedoptical brightener in addition to the mono alkali metal salt of citricacid.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 is a drawing of the work station for carrying out the second sidecockle test method.

FIG. 2 is a schematic diagram of a paper machine.

DETAILED DESCRIPTION

One embodiment of the present invention is directed to an uncoated orlightly coated paper having a mono alkali metal salt of citric acid andwhich may be used for printing on ink jet printers and which has amaximum Cockle Value of 0.25 after such printing. Another embodiment isdirected to an uncoated or lightly coated printing paper having a monoalkali metal salt of citric acid and which is water fast.

It has been found that a mono alkali metal salt of citric acid such asmono sodium or potassium citrate has enough cationicity to react withthe ink jet inks to cause them to be water fast. The mono alkali metalsalt of citric acid also causes the paper to have a Cockle Value of 0.25or less. The mono alkali metal salt of citric acid does not have enoughcationicity to react with an optical brightener, including hexasulfonateoptical brighters, or to react with offset printing inks. It provides apaper that is water fast, that has a reduced cockle and can bepreprinted in an offset

In another embodiment of the invention the paper also contains a bindersuch as starch, ethylated starch, latex, polyvinyl alcohol, styreneacrylic acid or an ester in addition to the mono alkali metal salt ofcitric acid and the low viscosity of the additive can be maintained.

In another embodiment of the invention the paper also contains aflorescent whitening agent, an optical brightener or a hexasulfonatedoptical brightener in addition to the mono alkali metal salt of citricacid.

A quantitative test has been developed to determine the curl and cockleof paper. It replaces the subjective test of viewing the paper todetermine whether there was curl and cockle and the amount of curl andcockle. This prior subjective test also determined whether a sheet ofpaper had sufficient treatment. The quantitative test is the second sidecockle test method.

The second side cockle test method is used to evaluate the amount ofcockle that an inkjet print, at an ink application level of 5.9grams/square meter, produces in the unprinted or second side of a paperprinted with a block print. The present test used a Scitex Test CockleForm Print. The unprinted side of the inkjet print is illuminated usinglow angle (15°) lighting. A digital image is made of the cockled area onthe unprinted side associated with a 3.5 by 3.5 inch half-tone printedsquare on the printed side of the sample. The image is then evaluated todetermine the amount of second side cockle.

The apparatus used for the second side cockle test method is shown inFIG. 1. It includes a test platform 10, a Kodak® megaplus 8-bit digitalcamera 12, and a Dedolight® light 14. The camera 12 is mounted abovesurface 16 of the test platform 10 and at 90° to the surface 16 of thetest platform 10. The camera is aimed directly at the center of thesurface 16 of the platform. The Dedolight light 14 is mounted at anangle of 15° to the surface 16 and also aimed at the center of thesurface 16. Mathworks, Inc. Matlab® computer software is used to analyzethe images.

The samples of paper to be tested are printed on one side with a ScitexTest Cockle Form using an inkjet printer and inkjet ink. In thefollowing tests a Hewlett Packard ink jet printer HP560C was used. Theink used was Scitex Ink 2002 and the ink application level was 5.9g/square meter. The ink should be a water based ink. The paper washandled carefully so as not to crease or wrinkle the paper becausecreases or wrinkles would be analyzed as cockle.

The settings of the camera 12 were adjusted to a pixel resolution of 100microns/pixel and an f-stop of F8. The camera control was on Fixed andthe image centering was at 127. The Dedolight light 14 was adjusted foruniform low angle lighting. All lighting was from the Delolight light14. Other room lights were turned off.

The paper sample 18 was placed on the surface 16 of the test stand 10with the unprinted side of the paper turned to the camera and facing up.The 3.5 by 3.5 inch cockle area was centered in the camera field of viewwith the light aimed at the center of the cockle area. The camera'sexposure was adjusted until the average image pixel value was 127. Theimage was collected and saved to a disk.

This process was repeated for each sample.

The images were analyzed using the Mathworks, Inc. Matlab® computersoftware. Version 6, release 13 was used. The image is read into theprogram and smoothed with a 5×5 median filter to remove high frequencynoise. The mean, standard deviation and coefficient of variation werecalculated for each row and column. The larger of the maximum rowcoefficient of variability and maximum column coefficient of variabilityis taken as the sample Cockle Value. The program is evaluating thedifferences between the light and dark areas of the image anddetermining the variability.

Cockle Value means the cockle value determined by this test.

An embodiment is an uncoated paper that has been treated with ahexasulfonated optical brightener and at least 50 pounds per ton ofpaper with a mono alkali metal salt of citric acid, a mono sodium ormono potassium citrate. The maximum Cockle Value of the treated printingpaper is 0.25. A ton is defined here as 2000 pounds. The salt of citricacid would be applied in a solution at a concentration of 20-50% of thetotal weight of the solution at room temperature or at temperatures of50° C. or less.

In another embodiment of the invention at least 75 pounds of citrate perton of paper is used. In another embodiment of the invention as much as300 pounds of citrate per ton of paper may be used. In anotherembodiment as much as 250 pounds of citrate per ton of paper may beused.

The citrate is applied at the size press or the blade coater. It may beapplied using a puddle, gate roll or metered size press, or a knife orblade coater. In one embodiment the citrate may be applied in a solutioncontaining at least 20% by weight of material. In another embodiment thecitrate may be applied in a solution containing 20 to 50% by weight ofthe material.

60 gm./m² unsized paper was used as the base paper for the sheets inthis example.

The percentages in this example are weight percentages.

A control sample of paper was coated in a laboratory size press withethylated starch at 12% concentration. Both side of the paper werecoated to a coat weight of 40 pounds of starch per ton of paper perside. This is typical of most uncoated paper grades (Formula I).

Each of the sheets were then dried and conditioned at 50% R.H.

Two commercial paper products A and B, printed with a HP 560 printerusing Scitex high speed ink jet ink were digitally recorded with a SONYMavica digital camera, under low angle light.

A sample of paper was treated in a lab size press with a solutioncontaining a concentration a 25% of the monosodium salt of citric acid(monosodium citrate), heated to 50 degrees C. Both side of the paperwere coated to a coat weight of 37.5 pounds of material per ton of paperper side (Formula II).

The sheets were then dried and conditioned at 50% R.H.

One set of the sheets was printed using an HP 560 printer and ScitexHigh Speed ink jet ink. The image was a 3″×3″ square, printed at 60%density, using Corel Draw, Version 10.

The printed sheets were then placed in a darkroom, face down and viewedunder a LANDSCO triple-bulb, low angle light. The degree of curl andcockle were then visually estimated. The sheets were judged against thestarch control. The results are shown in Table 1. 100% is the base casefor a starch control.

The sheets were also tested for water fastness via submersion in waterfor 60 seconds and the ink dye was completely immobilized by the salt.The results are also shown in Table 1. TABLE 1 Degree of Curl/ Cockle:Water Formula Scitex ink Fastness Formula I 100% Poor Formula II   <5%Excellent

Water fast means the ability of ink to remain intact when exposed towater or moisture. Water fast inks do not bleed. Water based inks mustbe treated to be water fast. The inks are anionic. It has been foundthat the mono alkali metal salt of citric acid has enough cationicity toreact with the anionic ink and make it water fast. It does not, however,have enough cationicity to react with offset printing inks.

Water fastness is typically obtained with a nitrogen-containing organiccompound of a cationic nature and functions by precipitating the dye inthe ink, rendering it immobile, when exposed to moisture after printing.Unfortunately, these types of materials are incompatible with anionicfluorescent whitening agents, optical brighteners, or hexasulfonatedoptical brighteners which are typically applied at the size press tobrighten paper. As such, these types of additives reduce the overallpaper brightness, often times to levels below customer acceptance.Again, the mono alkali metal salts of citric acid, such as sodium andpotassium citrate, do not have enough cationicity to react with theoptical brighteners, the flourescent whitening agents or thehexasulfonated optical brighteners.

The pH of the mono alkali metal salts of citric acid is acid enough toreact with the ink jet inks but not acid enough to react with theoptical brighteners or the offset printing inks.

Monosodium citrate maintains the brightness of the paper withfluorescent whitening agents, optical brighteners or hexasulfonatedoptical brighteners while providing water fastness and a low degree ofcurl and cockle.

Water based inks, those found in ink jet printing, are anionic. Waterbased inks will bleed when placed in water. The inks must be fixed byprecipitating the dye in the ink. This is usually done with highlycationic fixatives. The mono alkali metal salt of citric acid is acidenough to react with the ink jet inks but not acid enough to react withthe optical brighteners or offset printing inks.

Samples were also evaluated using the second side cockle test method.

The percentage shown are weight percentages.

60 gm./m2 unsized paper was used as the base paper for the sheets inthis example.

The Formula 1 control samples were also used as the control samples.

A second sample of paper was treated in a lab size press with a solutioncontaining a concentration a 25% of the monosodium salt of citric acid,heated to 50 degrees C. This percentage is a weight percentage. Bothside of the paper were coated to a net coat weight of 37.5 pounds ofmaterial per ton of paper per side. The amount of citrate was 37.5pounds per ton of paper per side.

Two commercial papers were added to the study.

The samples were evaluated both visually and using the second sidecockle test method. The image evaluation test correlated well with thevisual observation. The results are given in Table 2. TABLE 2 FormulaSubjective Value Cockle Value 25% mono sodium Acceptable 0.2296 salt ofcitric acid 12% Starch control Unacceptable 0.2658 First Choice ™Unacceptable 0.2851 CI-2000 ™ Unacceptable 0.3211

It was determined that paper sheets having Cockle values of 0.25 or lesswere acceptable.

FIG. 2 is a schematic drawing of a paper machine. Wood pulp fiberfurnish and wet end chemicals are mixed with water in a headbox 20 toform a slurry. The slurry exits the headbox through a slice 22 onto awire 24. The water in the slurry drains from the wire. A vacuum chest 26is also used to draw water from the slurry to form a wet paper web. Theweb is carried through press rolls 28 and a drier 30 that removeadditional water.

Additional size press chemicals or materials are placed on the wet paperweb at the size press 32. The size press may be a horizontal type withthe rolls horizontally aligned, a vertical type with the rollsvertically aligned. The materials may be placed on the web from therolls or from a puddle between the rolls. The web may, in someinstances, be coated with material by the spraying apparatus 34. Thematerials described in the various embodiments in the presentapplication would also be applied at the size press 32 or the sprayingapparatus 34.

The paper web then passes through a drying section 36. The drying isusually done by steam heated drier cans through which the paper web isthreaded. The paper is then calendered by calender rolls 38 and rolledinto paper rolls at the winder 40. The resulting product is known asuncoated paper.

This is the product of the present invention. Additional expensiveoff-machine coatings would not be required to provide a paper that has amaximum Cockle Value of 0.25.

Those skilled in the art will note that various changes may be made inthe embodiments described herein without departing from the spirit andscope of the present invention.

1. A method of manufacturing uncoated printing paper comprising forming a wet web of cellulosic fibers, applying to the web at least 50 pounds of a mono alkali metal salt of citric acid per ton of paper, applying-to the web hexasulfonate optical brightener, drying the web to provide an uncoated paper having water fastness.
 2. The method of claim 1 wherein the salt of citric acid is mono sodium citrate.
 3. The method of claim 1 wherein the salt of citric acid is mono potassium citrate.
 4. The method of claim 1 further comprising applying to the web prior to drying a starch, ethylated starch or polyvinyl alcohol.
 5. The method of claim 4 wherein the salt of citric acid is mono sodium citrate.
 6. The method of claim 4 wherein the salt of citric acid is mono potassium citrate. 